Replaced fopen every trace call by a single fopen when tracing/dumping data

Change-Id: If752b2db0f3bd7125508b88f32248af6fa3c598d
Reviewed-on: https://swiftshader-review.googlesource.com/2340
Tested-by: Maxime Grégoire <mgregoire@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>